Field
The present invention relates to a speed change apparatus for a vehicle.
Description of the Related Art
Conventionally, in relation to a transmission for a motorcycle provided with an AMT (automated manual transmission) transmission, there has been disclosed a peripheral structure for a shift spindle (see, for example, PCT Patent Publication No. WO 2014/157631 (Patent Document 1)). Specifically, the shift spindle in the document is of a so-called interlocked spindle type provided with both a clutch lever for operating a clutch and a master arm for operating a change mechanism of the transmission. Besides, a force accumulation mechanism is provided between the shift spindle and the master arm. Here, the shift spindle is driven by a shift motor.
The change mechanism includes the master arm, a shift drum rotated by the master arm, and a shift fork which is driven by the shift drum and moves in an axial direction a shifter gear constituting a part of a row of gears in the transmission. A stopper pin is provided at a wall portion of a power unit case, and a return spring for biasing the master arm so as to return into a neutral position is provided in a locked manner between the master arm and the stopper pin.
The force accumulation mechanism includes a gear shift arm which is provided on the shift spindle in a relatively rotatable manner and rotates the master arm, a force accumulation collar which is rotated as one body with the shift spindle, and a force accumulation spring which is provided between the gear shift arm and the force accumulation collar.
In the motorcycle provided with such a force accumulation mechanism, when a shift signal is outputted from a controller of the AMT during running of the vehicle and the shift spindle is rotated, the gear shift arm receives a load in a direction in which it is rotated through the force accumulation spring from the force accumulation collar. However, until the clutch lever is rotated and the clutch is disengaged, a frictional force due to a driving force for running is acting on dog clutch tooth side surfaces (driving force transmitting surfaces) between a shifter gear and a free gear in the row of gears in the transmission. Therefore, the shift fork cannot move the shifter gear. For this reason, before the clutch is disengaged, the gear shift arm does not rotate the master arm and the shift drum, even if the shift spindle is rotated; thus, only the force accumulation collar and the force accumulation spring are rotated, whereby load is gradually accumulated in the force accumulation spring. Thereafter, when the clutch lever disengages the clutch, the frictional force acting on the dog tooth side surfaces in the row of gears in the transmission is released, so that the gear shift arm, the master arm and the shift drum are quickly rotated at a stroke by the load and rotational angle having been accumulated in the force accumulation spring. As a result, the time required for a gear change can be shortened.
A problem to be solved relates to a return torque of a shift spindle and a torque which is applied to a shift drum by a master arm. First, a torque applied to a shift drum by a master arm in a related art example will be described.
The torque applied to the shift drum by the master arm is determined by torques exerted on a force accumulation mechanism, a return spring, a shift spindle, and a shift motor. Here, description will be made along a downstream course of a transmission path of a driving force in a change mechanism. For simplicity of explanation, frictions on component parts (frictions in bearing portions, etc.) are neglected. A shift motor is disposed at the most upstream end of the transmission path of the driving force in the change mechanism. As shown in FIG. 21, a torque exerted on the shift spindle after the speed of rotation of the shift motor is reduced by a speed reduction gear is referred to as TMO. The relation of a torque TSD, which is applied to the force accumulation spring downstream of the shift spindle by the shift spindle, with TMO is TSD=TMO. The relation of a torque TCR, which is applied to the master arm downstream of the force accumulation spring by the force accumulation spring, with TSD is TCR=TSD. Not only the torque from the force accumulation spring but also a reverse-direction torque Tmr from the return spring is applied to the master arm. Therefore, a torque TMA exerted on the shift drum downstream of the master arm by the master arm is TMA=TCR−Tmr. The speed at which the change mechanism is moved is higher as the torque TMA exerted on the shift drum by the master arm is higher.
Note that after an operation of the master arm is started (after an operation of the shift drum is started), “dog abutment” may be generated in which a top surface of a dog tooth of the shifter gear being moved comes into abutment on a top surface of a dog tooth of the free gear on the intended shift position side (for dog abutment, refer to Japanese Patent Laid-Open No. 2014-199102). Where dog abutment is generated, the dog abutment will be soon canceled by a difference in rotational speed between the shifter gear and the free gear on the intended shift position side. Although the dog abutment is soon canceled, the time taken for canceling the dog abutment during the driving force non-transmission period is prolonged, and the time taken for the dog teeth to move in the axial direction for meshing with each other (the time necessary for a reliable shift) is shortened accordingly. In some cases, “shallow engagement” may be generated in which the dog teeth are meshed in a shallow depth of mesh. While dog abutment is generated on a probability basis, in order to restrain the generation of dog abutment, it is desirable that the difference between the torque of the master arm and the torque of the return spring at the time of “gear-in” be not less than a predetermined value (TES), as shown in FIG. 20.
Here, in the transmission as in the related art example, it may be desired, depending on the model of the transmission, to enhance the load of the return spring. If it is attempted to simply enhance the torque Tmr of the return spring and secure the above-mentioned predetermined value TES as shown in FIG. 22 in a system according to the related art example, the increase in Tmr makes it necessary to enhance a preload of the force accumulation spring or enlarge the thickness (diametrical size) of the force accumulation spring so as to enhance the spring constant. Then, the torque TMA′ exerted on the shift drum by the master arm would be higher than that in the related art example of FIG. 20. An increase in TMA′ leads to a corresponding increase in the rotational speed of the master arm as aforementioned, and the increase in the rotational speed of the master arm would lead to another problem in that the collision sound (impact sound) generated when the rotated master arm comes into contact with the stopper pin is enlarged.